Our invention relates to communication systems and more particularly to the automatic conditioning of speech and other other information signals for transmission.
In speech and similar type communication systems it is often desirable to automatically adjust the input speech so that signal transmission over a prescribed communication channel is optimized. Signal amplitude equalization, for example, is effective to reduce the dynamic range of the transmitted signal whereby the level of relatively weak signals is raised to that of a relatively strong signal. Such conditioning results in reduced vulnerability to noise. Where the communication signal incorporates an amplitude modulated transmitter having limited peak modulation capability, automatic signal amplitude equalization increases the average level of the equalized modulating signal to a high level within the modulation capability of the transmitter so that greater side-band power is available.
In one type of priorly known automatic signal amplitude adjustment arrangement, the input signal is filtered to obtain a control signal representative of the long-term signal amplitude level. The control signal is then used to adjust the input signal amplitude. Such arrangements are effective to equalize signal amplitude over syllabic or longer time intervals, but are relatively ineffective over shorter time intervals. Another well-known amplitude control arrangement utilizes peak clippers, instantaneous compandors, or non-uniform or adaptive quantizers to effect control over very short duration signal variations but the degree of equalization is limited and longer term signal changes are not fully compensated. Yet another well-known amplitude equalizing scheme uses linear filtering with high frequency emphasis to minimize the amplitude differences between lower frequency voiced speech segments and higher frequency unvoiced speech segments occuring in intermediate range time intervals. All of these priorly known arrangements, however, include time-dependent or frequency-dependent control functions which limit the range of automatic signal amplitude adjustment.
It is an object of the invention to provide improved signal amplitude equalization operative over an extensive range of time intervals.
It is another object of the invention to transform an input speech-type signal into an intelligible, substantially unilevel signal of low distortion.